Recently varieties of metallic materials have come to be utilized and increasingly severe demands are being made regarding the quality of these materials. Thus control of the concentrations of impurities which have a close relation to the quality of the product is now considered to be of great significance in processing molten metal. Inter alia , the determination of oxygen potential in molten metal is very important in the control of the concentrations of oxygen and impurity elements which are in equilibrium with oxygen. A typical example is the determination of oxygen potential in molten steel and copper.
As a means for determining oxygen potential in molten steel and copper, zirconia oxygen sensor- is known, which utilizes the electromotive force of solid electrolyte. However, such an oxygen sensor can be used only in a rather high temperature range such as 1500-1800.degree. C. for steel and 1000-1300.degree. C. for copper.
It has been commonly accepted that the known zirconia oxygen sensor used for molten steel and copper cannot be used for determination of oxygen potential in a low-melting metal such as zinc, lead, bismuth, indium, gallium and alloys thereof whose melting point is not higher than 550.degree. C. and there are hardly any reports of such measurement being conducted. It has been considered that the measurement of electromotive force itself is difficult at a low temperature such as 450-500.degree. C., in molten zinc for instance, since the oxygen ion conductivity of the zirconia sensor is small at such a low temperature.
As an oxygen sensor for low-melting metals, an oxygen sensor, which utilizes thoria solid electrolyte and can be used in liquid sodium, is disclosed in Japanese Laid-Open Patent Publication No. 52-53494. However, thoria used in this sensor is a radioactive substance which is expensive and very difficult to handle and thus this sensor cannot easily be used in industry. That is to say, there has been known no oxygen sensor which can determine oxygen potential in low-melting metals.
As the reference electrode for a zirconia oxygen sensor, solid electrodes of Cr--Cr.sub.2 O.sub.3 system, Mo--MoO.sub.2 system, etc. are used for molten steel and those of Ni--NiO system, Fe--FeO system, etc. for molten copper. These electrodes exhibit rapid response to electromotive force at temperatures of 1000-1800.degree. C. but they require an impracticably long time before the electromotive force stabilizes or otherwise the electromotive force does not stabilize when they are used in a low-melting metal which melts at a temperature of 350-550.degree. C. We have now found that the oxygen potential in a low-melting metallic material can be measured by using a zirconia oxygen sensor comprising a reference electrode of a system consisting of a metal which has a melting point of the same level as or lower than that of the low-melting metal and is liquid at the measurement temperature and an oxide thereof, a zirconia solid electrolyte and a lead wire which connects the reference electrode and the molten low-melting metal.